Simons Observatory

We are building a series of telescopes to collect photons from the Cosmic Microwave
Background (CMB), the most ancient light in the universe. The CMB is the relic radiation
that began propagating freely shortly after the big bang, nearly 14 billion years ago.
These primordial photons can answer questions in subjects ranging from cosmology and
fundamental physics to the astrophysics of distant objects.

The properties of the CMB tell us about the condition of the universe at the time that
this light was first emitted. By looking at the CMB today, we learn about the distant,
ancient universe; this is the beauty of CMB cosmology.

Along their path to our telescopes, some of the CMB photons encounter the large
structures
forming in the evolving universe. These structures gently change the path of the photons
as
they travel towards us. By tracking these
modifications, we can learn about the properties of the objects along the photons’
paths;
this is CMB Astronomy.

The very peculiar way in which CMB photons are distributed in the sky is also the result
of
fundamental interactions between the basic components of our universe. These
interactions
are set by the fundamental forces in physics. By carefully reconstructing this
distribution
of where the photons are on the sky, and the properties that they have, we can learn
about
these interactions; this is Fundamental Physics. The Simons Observatory telescopes will
be
designed and built to tackle these ambitious goals using light from the cosmic birth.

Searching for Inflation

It is widely believed that very shortly after the big bang the universe
went
through a period of rapid expansion. Simons Observatory will search for
direct evidence of this expansion.

When you look up at the sky, you might guess that patches of the sky far away from one
another have been evolving independently from each other. However, previous CMB
measurements have indicated that this isn’t the case. In fact, all regions of the sky
seem to know something about one another. No matter how far apart, each part of the sky
is almost exactly the same temperature. This would be a quandary for Einstein himself,
as his theory of relativity tells us that these parts of the sky are too far apart to
have had any communication since the beginning of the universe. In the 1980s, a model
called inflation was suggested to explain this surprisingly uniform sky temperature, it
says the universe we observe today comes from a single patch that was rapidly stretched
at early times. Although our current understanding of the early universe seems to be
consistent with this inflation model, we have not yet observed the “smoking gun” of
inflation. This is a primary objective for the Simons Observatory. The smoking gun that
we are searching for is B-mode polarization pattern of CMB photons, a particular pattern
which looks “swirly” to the eye. If we see this pattern, we will be able to draw
conclusions about the inflationary model of the early universe.

The Dark Sector

A variety of cosmological measurements (of which measurements of the CMB is included)
have provided us with evidence that ~96% of the composition of our universe is not like
the kind of matter we interact with everyday (baryonic matter: protons, neutrons,
electrons, etc.), but is instead dark matter (~23%) or dark energy (~73%).

Looking for New Particles (Dark Matter)

A number of cosmological observations (e.g. the velocities of galaxies in galaxy
clusters, galactic rotation curves, measurements of gravitational lensing; the energy
distribution of the CMB) show effects that would be produced if there was a lot more
matter than what is actually seen. The proposed explanation for these observations is a
new, yet undetected type of particle (or particles) deemed Dark Matter (DM). The nature
of DM remains one of the largest mysteries in physics today. The unprecedented
sensitivity of the new generation of CMB experiments will enable scientists to identify
and study small, subtle features in the CMB pattern, thus providing a probe into the
properties of DM that is complementary to those from ongoing particle physics
experiments.

One way that DM can be probed is through gravitational lensing. When an object is
observed through a magnifying lens its image increases in size and becomes distorted.
The same phenomenon happens in the cosmos, where the lens is represented by massive
structures and the distorted objects are galaxies and the background CMB pattern.
According to the Einstein’s theory of General Relativity, the light that propagates in
the universe feels the gravitational potential sourced by massive objects (stars,
galaxies, galaxy clusters) and in response its path is altered. This gravitational
lensing effect is one of the most promising tool that cosmologists can use for mapping
the distribution of total matter in the universe, especially the invisible dark matter.

The Accelerating Universe (Dark Energy)

The fact that our universe is accelerating as it expands posed a surprising challenge to
our understanding of the laws of physics when it was discovered nearly 20 years ago.
This fact is still one of the greatest puzzles in the universe and we haven’t really got
passed giving it a name, Dark Energy (DE). Understanding DE is one of the driving
scientific goals for a whole class of next generation cosmological observatories.

Though we have no concrete understanding of DE, two classes of phenomenological
explanations have been proposed to explain it. First are new models of the gravitational
interaction which alter Einstein’s General Relativity. The second proposal is that there
is another completely new class of energy which behaves fundamentally differently than
anything we have ever observed and pushes all things apart like a sort of anti-gravity.

By observing small distortions in the energy and trajectory of CMB photons as they travel
through the forming structures in the universe over billions of years, we can learn
about the mechanism driving the acceleration of our universe, possibly discovering if
either of the suggested explanations is likely to provide the correct picture of our
universe, or if we have to start from scratch!

Measuring Neutrino Masses with CMB

It’s a matter of scale: measuring neutrino mass from cosmology

You might have heard of protons, neutrons, and electrons, which are the building blocks
of the matter around you. However, you might not of heard of neutrinos, the electron’s
shy cousin. Neutrinos rarely interact with particles like the protons, neutrons and
electrons, and are therefore difficult to study. Even though physicists have known about
neutrinos for over 80 years, we still don’t know how heavy they are (i.e. we don’t know
their mass). We do know that they have a (tiny) mass from experiments that have observed
neutrinos changing their flavour along their path from the emitting source to the
observer (neutrino oscillations). This phenomenon is only possible if neutrinos have a
non-zero mass. Neutrinos are the only standard model particles of unknown mass (and we
really want to fill this gap!) and measuring the neutrino mass scale is a fundamental
step towards unveiling other properties of these unique particles.

Dark Matter (which is partly made up of neutrinos) is the stuff that primarily dictates
how the universe’s constituents clump together on very large scales. If neutrinos have a
large mass, they will make up a large fraction of the DM and if they have a small mass
they will make up a small fraction of the DM. If a large fraction of the DM is made up
of neutrinos, then we expect that the large scale stuff in the universe to look washed
out; however if a small fraction of the DM is made up of neutrinos, we expect the large
scale stuff in the universe to look very clumpy. So, by looking at the large scale
structure of the universe, or how matter clumps together on the universe’s largest
distance scales, we can deduce what fraction of dark matter is composed of neutrinos and
eventually their mass.

Evolving Galaxy Clusters

Tracking the Giants: Understanding the Evolution of Galaxy Clusters

Some of the CMB photons we will collect have traveled through galaxy clusters (large
groups of stars, galaxies and hot gas bound together by gravity). Inside the clusters,
CMB photons scatter off electrons in the hot gas and increase their energy by a tiny,
yet detectable amount. The change in energy of these photons has a very peculiar
signature, and is called the Sunyaev-Zeldovich effect (or SZ effect).

Detailed analysis of the SZ effect for a particular galaxy cluster can teach us about the
properties of that cluster. In particular, we can use this to learn about the gas
composition and velocities of individual galaxy clusters, probing some of the largest
distance scales in the universe. If we combine the information from our SZ studies with
other observations of the same clusters, we can learn about about how these galaxy
clusters evolve.

Additionally, we can use our SZ information from many clusters to obtain a statistical
description of the distribution of clusters depending on their mass and distance from
us. This will give us information about the evolution of our universe and the
interactions at play between the different components of the cosmic inventory.